Low-Resistivity Ti/Al/TiN/Au Ohmic Contacts to Ga- and N-Face n-GaN for Vertical Power Devices

Low-resistivity Ohmic contacts to Gallium-face (G-aface) and/or Nitrogen-face (-Nface) n-GaN are undoubtedly needed for high-quality vertical power devices. Contrary to Ga-face n-GaN, for which the Ohmic contact formation is a well-established process, the formation of low-resistivity Ohmic contacts to N-face n-GaN is much more difficult due to the different surface properties. Despite many efforts, obtaining low-resistivity ohmic contacts, with rho(c) < 1 x 10(-5) Omega cm(2) to N face n-GaN, still remains difficult to achieve. Herein, results of fabrication and characterization of Ti/Al/TiN/Au ohmic contacts to Ga- and N-face n-GaN for vertical power devices are presented. The optimum annealing temperature is examined and it is noticed that Ohmic contacts to Ga-face n-GaN (n approximate to 5 x 10(18) cm(-3) , 200 nm on bulk GaN substrate) are formed after 550 degrees C annealing and continuously lower their rho(c) up to temperature of 750 degrees C for which rho(c) is approximate to 1.86 x 10(-6) Omega cm(2). Ohmic contact to N-face n-GaN (bulk ammonothermally grown substrate, rho approximate to 10(-3)-10(-2) Omega cm(2)) becomes quasilinear after annealing at 650 degrees C and becomes linear after annealing at 700 degrees C, reaching a minimum value of rho(c) 6 x 10(-6) Omega cm(2). Further annealing at higher temperatures increases the contact resistivity to the value of approximate to 5.8 x 10(-5) cm(2) for annealing at 800 degrees C. The electrical measurements results were accompanied with the results of structural characterization for determination of Ohmic contact formation mechanism.